Telecommunication equipment connected to exposed metallic communication lines is in most cases protected against destructive overvoltages caused by e.g. lightning. The overvoltage protectors must have a high surge current capability, and they are not allowed to have any electrical properties that can disturb the communication signals on the lines. The only overvoltage protection component that fulfils both requirements is the gas discharge tube, which is used in many overvoltage protection circuits for telecommunication equipment. In particular gas discharge tubes are used as overvoltage protectors on high speed communication lines because they do not affect or change transmission characteristics of the lines. The reason is the very low capacitance and very high isolation resistance of a gas discharge tube when compared to semiconductor type protectors.
One known problem with gas discharge tubes is, however that they when exposed to repeated overvoltage transients, degrade and finally stops working. The degradation appear as a gradually increasing break down voltage, which finally becomes so high that the gas discharge tube stops functioning as an overvoltage protector, and starts behaving like an open circuit with a very high resistance, about 1000 Mohm, connected between the signal line and ground. The telecommunication equipment is now unprotected against overvoltages, and since an open circuit does not disturb the communication line, there is no indication that anything is wrong with the overvoltage protection until the telecommunication equipment is destroyed by overvoltages.
This degradation is impossible to measure while the overvoltage protector is connected to the communication line, since the high test voltage would cause communication errors on the line, and if the gas discharge tube is broken and does not spark over, the high test voltage could even cause damages in the communications equipment connected to the line.
One way of limiting the problem mentioned above is to perform regular maintenance and physically remove all gas discharge tube from the communication lines and test them with a special high voltage gas discharge tube tester at least once a year.
Another way is to measure the breakdown voltage of the gas discharge tube during the voltage transients that are caused by lightning, and let the detector set an alarm if breakdown voltage exceeds a set level. In this case, the measurement does not cause any additional disturbances in the communication, since the communication is disturbed by the naturally occurring transients anyway. However, since a gas discharge tube has a breakdown voltage that is dependent on the dv/dt of the overvoltage transient (how fast the voltage rises), the detector must adjust the breakdown voltage alarm level upwards for a fast dv/dt and downwards for a slow dv/dt.
A more practical solution is disclosed in U.S. Pat. No. 5,966,283, in which a detector is connected to the overvoltage protector circuit in order to detect when the overvoltage protector has degraded so much that it will fail to work as intended. The detector includes a high voltage gas discharge tube and a fuse connected in parallel with the protector gas discharge tube and to ground. The idea is that the detector gas discharge tube is selected to have such a high breakdown voltage that it only sparks over when the protector gas discharge tube has been degraded to a certain level.
When that occurs, an alarm circuit is activated to announce that the protector gas discharge tube needs to be exchanged. The circuit is arranged so that one pole of the alarm output port is connected through the high voltage gas discharge tube to the communication line on which overvoltage transients occurs. When the high voltage gas discharge tube sparks over for the first time, the fuse will be blown, and an overvoltage transient will be passed to the alarm output port. So far the solution works well but after the fuse has blown, the alarm output port is still connected to the communication line. Since overvoltages often come in series new ones can pass from the line and out through the alarm output port to the external alarm central and cause damage.